1. Field of the Invention
The present invention relates to an insulated gate field effect transistor and, more particularly, to a transistor structure which has its short-channel effect depressed to improve the source-to-drain breakdown voltage and the controllability of a threshold voltage so that it can be suitably used in an LSI having a high integration density.
2. Description of the Prior Art
In the insulated gate field effect transistor of the prior art, especially, in an MOS transistor, it is a current practice to form an impurity layer of the same conduction type as that of a substrate by a method of implanting ions into the surface region of a channel so as to raise a threshold voltage V.sub.th thereby to provide an enhancement type device. The MOS transistor of this type (which is called the "surface type") has its defect in that the so-called "short-channel effect" is high, e.g., in that the dependency of the threshold voltage V.sub.th upon the gate length is high and in that the source-to-drain breakdown voltage is low.
As the MOS transistor which has succeeded in eliminating that defect, i.e., in depressing the short-channel effect, there is known a transistor which has a higher impurity distribution in a higher region of a channel. This impurity layer formed in the deeper position is called the "punch-through stopper". More specifically, since the aforementioned surface type MOS transistor has its impurity layer existing in the surface of the channel portion, a depletion layer is liable to be formed in a deeper region than that impurity layer, and the depletion layer to extend from the drain is made liable to extend in that deeper region thereby to reach the source by the action of a voltage V.sub.D applied to the drain. Therefore, the depression of the elongation of the depletion layer in that region belongs to the function of the punch-through stopper.
As a result, the short channel effect is depressed, but the breakdown voltage of the drain junction is lowered as an adverse effect together with the source-to-drain breakdown voltage as a result of the fact that the impurity density in the vicinity of the drain is increased. Moreover, only the punch-through stopper finds it difficult to control the threshold voltage V.sub.th .
In any transistor having the channel portion of the construction thus far described, since carriers are allowed to move within a limited region of the surface of the channel portion, the carrier mobility takes a relatively low value.